Answer:
The selection that is changing bacteria in populations is natural selection. Bacteria populations can evolve through natural selection, which occurs when certain traits that confer a reproductive advantage are favored, and thus become more prevalent in the population over time.
In the case of bacteria, natural selection can occur through a variety of mechanisms. One example is the acquisition of antibiotic resistance. Bacteria that have a genetic mutation or acquire a resistance gene through horizontal gene transfer may be able to survive and reproduce in the presence of antibiotics, while bacteria that do not have the resistance gene will die off. Over time, the proportion of bacteria in the population that have the resistance gene will increase, leading to an overall increase in antibiotic resistance in the population.
Other factors that can drive natural selection in bacteria populations include changes in environmental conditions, competition for resources, and the ability to adapt to different niches or host organisms. In general, natural selection favors traits that increase an organism's ability to survive and reproduce in a particular environment or ecological niche.